1. Field of the Invention
This invention generally relates to a multi-lens system, and, in particular, to a lens system suitable for use in a copier. More specifically, the present invention relates to a full color copier lens system having an excellent correction in color aberration.
2. Description of the Prior Art
Recently, a demand for copiers has been rapidly increasing. In particular, over the last few years, a demand for full color copiers has increased. For this reason, there has arisen a demand for a new lens system suitable for use in a full color copier. As far as lens system is concerned, one of the largest differences between a lens system of a full color copier and a lens system of a monochromatic copier resides in that, in a full color copier, white light is decomposed into, for example, three color components, such as R, G and B, and these color component images must be focused on an imaging plane, such as the surface of a photosensitive member, on a one-to-one basis. For this reason, in the case of a full color lens system, it is required to minimize the longitudinal chromatic aberration as much as possible as compared with a monochromatic lens system. In a typical prior art monochromatic lens system for use in a copier, with a unity magnification arrangement, there is a longitudinal color aberration in the order of 0.01f (here, f: combined focal distance of a lens system). If such a lens system were used in a full color copier, the location of an image forming plane would differ among the color components of R, G and B by the amount corresponding to the longitudinal color aberration, so that there would cause a magnification error and a deterioration in image forming performance.
Japanese Pat. Laid-open Pub. No. 49-42335 discloses a lens system having a 6-unit-8-element structure. However, the lens system of this publication has the longitudinal color aberration for c and g lines of 0.01f or more and the F number of 10, which is rather dark. In addition, there are other disadvantages, such as large coma flare, excessive overall aberration and poor contrast.